1. Technical Field
The invention disclosed broadly relates to the switching of multiphase electrical power sources and more particularly relates to asynchronous multiphase switching techniques.
2. Background Art
Military power systems require frequent switching of the system load between several AC power sources. Aircraft switch from ground power to on board power prior to takeoff. The computerized electronic systems are often required to operate without power interruptions through this switching activity. Submarines, due to operational drills or casualty situations, need to alternate between their port and starboard power sources. This activity must not disrupt critical navigational, weapons, and sonar systems.
AC power sources cannot be paralleled unless the characteristics of the alternators are similar and operated at the same speed and voltage ratings. To allow paralleling, the power sources also have to be aligned electrically so that their respective phases can be tied together. If the speed or angle of rotation between each alternator is not exact, the machines will try to force each other into synchronism. This is an abrupt action as the rotating masses of the alternators cannot change speed instantaneously. The machines will nevertheless try to get into alignment as quickly as possible. The chances for the alternators to damage each other or physically break free from their moorings is very great especially when large unsynchronized machines are paralleled.
If the output voltages of the alternators are not equal, the one with the higher voltage will "hog" the load, making the sharing of demand difficult and causing poor utilization of the equipment. These difficulties cause the process of paralleling alternators to be a sensitive operation, especially with machines larger than several kilowatts. For this reason, system load transfers between power sources have required that the old power source be disconnected before the new power source is connected. This break-before-make switch causes power dropouts for significant time intervals, seriously impacting the system load.
Previously, reliance on fast recovery computer systems has tried to reduce exposure to this weakness on the platform. With low conversion efficiencies, conversion of the AC power to a common DC grid, and then inversion back to AC becomes prohibitive as the power handled increases. The batteries and switching equipment necessary to implement such a conversion/inversion occupy too much space and increase the overall weight greatly. Cooling the conversion equipment becomes an additional concern as the efficiency losses are given up as waste heat.
DC "auctioneering" or switching between two power sources is somewhat easier. However, redesigning equipment to operate off of DC power sources often requires great expense and renders older equipment obsolete. Control and switching of DC power also increases the complexity and size of hardware design and this impact limits the conversion of older equipment to DC.
What is needed is uninterrupted power switching provided by a new method not requiring synchronous power sources.